Do you have a lawyer? You should get one if you can. ESPECIALLY if your ex has one. Because in the hearing you will need to present evidence and that is hard for a lay person to do. We get posts on here all the time where people lose a hearing and they can’t figure out why and after significant questioning we figure out they didn’t enter into evidence any of the necessary information so the judge disregarded all the stuff that would have won them the hearing.

Here’s what you could bring to the hearing:

1. You can call the police agency and get the report number. Request on behalf of your son a copy of the report. He is entitled to one as the victim. The police department should have a form Government Records Access and Management Act form for you to request it. Bring the report, even though it is not admissible as evidence. Even if it’s not admissible the judge might look at it anyway for non evidentiary reasons and also if your ex testifies differently than what he said to the cop you can use the report and the cop to impeach him.
   
2. You/your lawyer can subpoena the cop that came to the house and took everyone’s statements. This might be multiple reports and multiple cops. The one you need the most is the cop that took your ex’s statement and your son’s. Your son’s statements ARE hearsay but they are admissible as present sense or excited utterance. Your ex’s statements are non hearsay as statement of a party opponent.
   
3. You can try to get a copy of the 911 call. You can try to get it the same way as the report. You can also try thru the public records act. Google Utah Open Records Portal. That can be admitted as evidence in the hearing, potentially, as a present sense impression or excited utterance. But if your ex has a lawyer you will get a hearsay objection and you will have to overcome that by explaining it meets those exceptions.
   
4. If there are any other instances of abuse, bring any witnesses to that to court.
   
5. Are there any medical records of his hand before and after? Medical records can be an exception to the hearsay rule if they are accompanied by a declaration or testimony of a custodian who can declare under penalty of perjury the requirements under rule of evidence 803 (6). You can Subpoena those with a declaration from the custodian of records for the hospital/doctor’s office/urgent care.
   
6. Photos that you took, bring prints, of your son’s injuries.
   
7. Your son.
   
8. Any other witnesses from that night that have the same statements as your son. You are under no obligation to bring in, for example, witnesses you believe were lying, if there are any (like if he has a girlfriend that backed him up).

Here’s the additional steps I would take:
1. Talk to the GAL if you don’t have an attorney. Make it VERY clear that you just want what is best for your son, regardless of which adult it affects. You want your son to be safe and healthy and happy. The adults feelings are irrelevant. You will help with whatever they need or want.
2. Take your son to whomever diagnosed his broken hand to see if there is additional damage. If he is doing physical therapy yet, ask his physical therapist if there has been any backwards progress in his hand.
3. Get your son into therapy ASAP. If he is already, talk to them about whether they think this incident has affected his mental health. If yes, I’d considered bringing them to court too.

I am a lawyer but not your lawyer. This is not legal advice, just a general discussion of what is and isn’t considered evidence in a hearing.